Altimeter



Dec. 109 1946u Pf F. sHxvERs ALTIMETER Filed Nov. 20, 1942 5 Sheets-Sheet l Dec. 10, 1946. P F, 5 H|VERS 2,412,541

ALTIMETER Filed Nov. 20, 1942 5 Sheets-Sheet 2 EW-L- @We 4 @M5L Gitorucg Dec. l0, 1946. P. F. sHlvERs ALTIMETER Filed Nov. 20, 1942 5 Sheets-Sheet 3 mMT.,

Cttorneg Dec. 10, 1946. Y P. F. sHivERs ALTIMETER Filled Nov. 2o, 1942 5 Sheets-Sheet 4 l@ wktsw Gttorneg Dec. 10, 1946.

,YLT/Taos @van -5 Sheets-Sheet 5 Patented Dee. 1o, e1am l 2,412,541

UNITEDy y STATES PATENT OFFICE 2,412,541 -AL'rlME'rER Paul F. Shivers, Edina, Minn., asslgnor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application November 20, 1942, Serial No. 466,283

an inverse logarithmic relation te 15 Claims.

My present invention relates to measuring in the magnitude of a variable condition by their response to related changes in the pressure of a means operatively connecting a pressure rean. instrument such as I have described, the connection being adjustable to compensate for difiergas mass. More specifically, my invention relates 5 ences, in density and temperature, between the' to the eld of altimeters which indicate variations standard atmosphere and the actual atmosphere in'altitude above a reference point by responding between the instrument and the reference peint. to the corresponding variations in the static air Another object of my invention is to provide pressure (that is, the barometric pressure). novel means, in an instrument such as l'. have The static pressure indicated by such an in- 10 described, for increasing or decreasing the altitude strument is influenced by the altitude of the inindicated by the instrument in accordance with strument and by the density of the air mass and increase or decrease of the density of the animeea its temperature gradient. For the saire of unphere between the instrument and the reference formity in instrument dial calibration, certain pointarbltrary values of density and temperature have l5 A further Object or" my invention is te Urol/ld@ been selected to define a "standard atmosphere." novel means, in instrument Such as' have rlhis standard atmosphere is of suoli density as to described, for adjusting the instrument t@ 0121 give a barometric pressure at point one tout pensate L50? dffels bJWEe the 2li-@2m GII m above sea level equal to that oi? a column of merperature of the standard atmosphere cury 29.92 inches high. The temperature mean temperature of the atmosphere between gradient in the standard atmosphere is -sucli that the instrument im@ the Teen@ Swimat sea level the temperature is F., while te@ Yet another obiect of my invention is to are temperature is 67 at the bottom si an 35;; vide novelineanain aninstrurnent suoli as b thermal layer which has been found to exist at describen OT lls'tls the lliil Gl a variable height of approximately 35,@@0 m instrument in accordance with variation in middle latitudes. .ln this standard atmosphere the barometrie pressure has been :found to have the altitude, and pressure responsive means have been designed. to give indications'on a linear scale resp@ to logarithme elianges in pressure, e altitude on this scale corresponding to given static pressure is called the pressure altitude the point at which the pressure indication is ense-riferi, and is the altitude indicated by uncompensated altimeter.

The atmosphere enveloping craft :lT not normally thi s but differs i may be caused. by Variations of t,M

from the standard.

is an object of my invention to gravide, in Ji lle a pressure responsive instrument for measuring 5S referente .I )J I? r. :e A a. altitudenovelmeans loradiusting tneinstrument met et. my invention is to nio to compensate for tite differences between the standard atmosphere and the actual atmosphere between the instrument and a reerence point..

It is another object of my invention to provide temperature at the reference point.

A still further object of my provide novel in en insturne have described, for adjusting the i instrument simultaneeusl llave separately onerabi eentrel means for elec adjusting an'aitimeter for air ternperatrue pressure tite referente peint and for responu sltely ticle rea-lne of the altimeter is alsoan object of my invention to pro- CII of the aneroid, the indicator means of the altimeter may be actuated and the aneroid returned to the original or zero position so as to eliminate insofar as possible mechanical or moving parts through which mechanical error might arise.

- Further; it is an object of my invention to provide control means for adjusting or varying the return or zero point of a. counterbiased aneroid for aI given ambient pressure, such control means being regulated `by a temperature responsive means.

Another object of my invention is to provide a pressure responsive altimeter and electrically operated means for the automatic correction of the indication of the instrument for the -iniluence of changes in atmospheric temperature.

It is also an object of my invention to provide in an altimeter a. plurality ofv photoelectric cells and a plurality of separate means for varying the relative electronic emissions therefrom so as to regulate the zero or return point of a counterbiased atmospheric pressure responsive member Other objects and advantages of this invention are set forth in the following description, taken with the accompanying drawings, and the novel features thereof are pointed out in the appended' claims. The disclosure, however, -is illustrative only and I may make changes in detail, especially in matters of shape, size and arrangement of parts within the principle of the invention, to the full extent indicated by the broad and Vgeneral meanings of the terms in which the appended claims are expressed. i

In the accompanying drawings wherein like reference characters indicate corresponding partsin the various gures, K Y A Figure 1 represents diagrammatically one form of my invention,

Figure 2 is an enlarged end view of the temy perature compensating control means shown in Figure l,

Figure 3 is an enlarged end view of the atmos- Y pheric pressure responsive control member of 'Y Figure 1.

or aneroid for a given ambient pressure, one of said means being controlled by the atmospheric pressure responsive member and the other of said means being controlled by a temperature responsive means.

Another object of my invention is to provide an instrument which will utilize an outside sourceV of power for .operating the gear train of the indicating mechanism, instead of utilizing the relatively small movement and power of the atmospheric pressure responsive member or aneroid, and in addition to so arrange the pressureresponsive, member Vor aneroid that it will have to work through only a very small range oi movement.

Another object of my invention is to provide control means for adjusting or varying the return or aero point of the aneroid or atmospheric pressure responsive member of my invention, for

v a given ambient pressure, whereby the ratio between barometric pressure and indicated altitude may be increased or decreased.

Another object of my invention is to provide control means for varying the operative standard or translation factor oi an instrument such as l; have mentioned in accordance with variations in atmospheric conditions.

A further object of my invention is to provide, in an altimeter, a plurality of photoelectric cells subjected to illumination from a source of light and control means adapted to regulate the illumination of the photoelectric cells whereby to cause actuation of the indicator means of the altimeter.

An additional object oi my invention is to pro,- vide, in an altimeter, a balanced bridge circuit and adjustable means for unbalancing the circuit so as to cause actuation of the altimeter indicator means.

Figure 4 represents a modied form of manually operable .temperature compensating control means.

Figure 5 is a front elevational view of the ccn- Vtrol panel of 'my invention shown in Figure 1 with the manually operable temperature compensating control means of Figure 4.

Figure 6 is a pictographical representation illustrating the variation in indicated pressure altitudes from an assumed standard altitude due to changes in atmospheric pressure caused by variation in the temperature of the mass oratmospheric air column above and below the assumed standard temperature. Y

Figure '1 represents a circuit diagram of a system embodying a second form of my invention.

Figure 8 is a front elevation of the control panel for the form of my invention shown in Figure'l.

Figure 9 represents a circuit diagram illustrating a furthermodifled form of my invention.

Figure 10 is a front elevation of the control panel for the form of my invention shown in Figure 9.

Figure 11 illustrates graphically the eect on barometric pressure of variation of the actual mean temperature from an assumed standard mean temperature as the altitude increases and showing that the greater the rate of temperature drop per unit change in altitude the lower the barometric reading for any given altitude.

In each of the curves of Figure 11 the ground barometric pressure at sea level is assumed to be 760 millimeters of mercury. Starting with this value, the line designated by the letter A indicates the relationship between altitude and barometric pressure under a hypothetical condition wherein the temperature of the air is constant at all levels. The line indicated by the letter B indicates the relationship between altitude and barometric pressure under a second hypothetical condition wherein the temperature of the atmospheric air at sea level is assumed to be 15 derived from report No. 538 of tions most generally assumed as standard in the calibration of the conventional altimeter: the temperature ofthe atmospheric air at sea level is assumed to be +15 degrees centigrade and for ,every 100G-foot increase in altitude the temperature decreases substantially 2 degrees centigrade until the isothermal layer is reached at which point the temperature of the air is -55 degrees centigrade. -Under such assumed standard conditions the air column has a mean temperature of -20 degrees centigrade. The line D indicates the relationship between altitude and atmospheric barometric pressure under a hypothetical condition wherein the temperature of the atmospheric air at sea level is assumed -to be +145 degrees centigrade and there is a decrease in the temperature of the atmospheric air of 5.66 degrees centigrade for every 1000-foot increase in altitude until the isothermal layer is reached with a temperature of -55 degrees centigrade. The latter air column has a mean temperature of +45 degrees centlgrade.

The computations'of the aforenoted graph are the National Ad-4 visory Committee for Aeronautics and the altitude pressuretablescontained therein based on the United States Standard Atmosphere. vIirom a study of the foregoing graph it will be seen that the greater the rate of temperature decrease with change in altitude the lower the barometric reading for any given altitude. Thus in an atmosphere where the rate of temperature decrease with change in altitude is greater than that in the standard atmosphere, the oarometric pres sure at any level is greaterthan that in thegstandarti atmosphere, and an uncorrected altimeter iridicates too low all altitude. Similarly, in an at= aerosols-ere where the rate oi temperature decrease,

with change :in altitude is less than iii the standard osphere, an unoorreeted altireeter inrlieates too leigh en altitude.

lilly invention embodies means for automa@ loa y' eorreoting saisi erroneous iiie'hoations of an ait meter so that the instrument willi give correct altiti indieetions regardless of whether tempes@ litioos or" the ambient o1 re e n 'or diverse from those et the sl atmosphere.

" of my invention means if), and ii, latter of? oeil S 5 and. c impe reses to constitute two aegee-exit arms ci? a bridge circuit indicated generaliy by the ninnerai de, which the remaining two atiiaoent arms comprise the' halves secondary winding ierroinets it and itl ot secondary eet bridge circuit and a pair of, output conductors il and l1 are connected respectively to center tap Il and to a terminal 20 on conductor H. Center tap Il and terminal 20 constitute the output ter-` minals of bridge circuit 35, and are connected by output conductors II and I1 with the input terminals 2| and 22 of a thermionic amplifier indicated generally by the numeral 39, and having output terminals 23 and 24. This bridge circuit is designed so as to be balanced when the amounts of light supplied to the cells bear a predetermined ratio to each other. Any decrease under the predetermined ratio of the light supplied to cell 5 to that to cell 6 causes a proportional increase in the resistance of cell 6, unbalancing the bridge circuit in such fashion that a potential diierence appears between terminals 20 and I8 having the same polarity and time-phase relation as the potential between terminals Il and I 6, and of a magnitudeproportional to the unbaiance of the bridge. This potential difference is fed through conductors I5 and Il to input terminals 2i and 22 of ampliiler 39.'

Further, an increase over the predetermined ratio of the light supplied to cell 5 to that to cell 6 causes a proportional decrease in the resistance of cell t, unbalanciiig the bridge circuit in such fashion that a potential difference appears between terminals liti and i6 having the same polarity and time-phase relation as the potential clierence between terminals it and it, and also o1? a magnitude proportional to the unbelance ol" 'the bridge. This potential clilerence is also fed. directly to terminals 2i and 22 ci amplifier Since instantaneous potential difference between terminals and i@ is out or" with between terminals ansi it will seen. the "eiitial diierence impresse -f across terminals and 22 oi amplifier 39 t l resistance of cell is decreased. is oi t e e phase to that impress-eel on the e 'when resistanse of ne electronic amplifier 'oh as sie 25, i923,

i l ce secondary N y f 'l n lne ane senses 7 As described in detail in the co-pending Upton application previously referred to, amplier 39 is of a type which produces in the output circuit including conductors 26 and 21 and motor winding 33 an alternating current of the same frequency and phase as the potential applied to input terminals 2| and 22. Since transformer primary windings 29 and l2 are both connected to the same alternating current supply lines |3 and |4, it will be understood that the potentials appearing at the terminals 'of the secondary windings 30 and 1 are of the same phase. The current liowing through winding 32 is, however,

shifted substantially 90 electrical degrees with l respect to this potential by the condenser 38, as

phase, the motor 3| is caused to rotate in a predetermined direction, depending on which of the currents lags and which leads.` The operation of a split phase motor of the type described herein is well .known in the art and is described` in lthe co-pending Upton application previously referred to.

The motor-3| is arranged to drive a shaft 40 in a direction depending upon the sense of -the unbalance of the bridgecircuit 35 and the resultant phase oi the current supplied from the amplifier to the motor 3|. The shaft 40 is, in turn .adapted bysuitable means such as a gear train conventionally indicated at 4I to drive an indicator means 43, which isof a suitable type well known in the art. Moreover, the motor 3| is further adapted by suitable means such as Y correspondingly slight.

the shaft 40,' train of gears 4|, and a further i shaft to turna cam 46 which cooperates with a roller member 1| mounted on a loading spring 41 for the adjustment` of the tension in the spring 41.V The loading spring 41 is pivoted at` one end on a knife edge 12 4and operably engages at the opposite end ananerold 65, exerting a com# pressive force on the aneroid 65 dependent in magnitude upon the adjustment of the tension graintained in the loading spring d1 by the cam As shown in Figure 1, there is mounted between the photo-electric cells 5 and 6 an illuminating lamp 5 l, energized through a suitable electric circuit this lamp provides the sole source o light for the cells. Suitable means are provided such as lenses E32 and 53 for directing the light rays from the lamp Eil to the respective photo-electric cells 6 and 6g. T

in order to control the supply of light to the photo-electric cell 6 suitable control means are provided which, as shown in Figure 3, may com-y prise' an adjustable member 66 having provided therein an aperture 62, for the passage of the light rays from the lamp 5i to the photo-electric cell 6, anda control slide 63 for regulating the supply of light passing through the aperture 62 to the photo-electric cell The control slide 63 is voperatively afxed at one end to a resilient bellows or other pressure responsive member 65 which may conveniently be referred to as an aneroid, 65 and the loading sL ing d1. The expansive force exerted by the aneroid 65 is counteracted by the atmospheric pressure acting upon the aneroid 65 and by the force exerted on the aneroid 65 by the spring 41. Spring 41 is operatively adjusted by cam 46, in response to motor 3|, so as to increase or decrease the force exerted by spring 41 upon the aneroid 65, as .previously explained.

- The member justed by a vernier adjustment knob 11 mounted on an instrument panel 54 whereby themeinber 64 may be move-d linearly vwith respect to member 63, whereby to vary theportion of aperture 62 covered by member 63. Knob 11 has a pointer l89 for cooperation with suitable scale markings ycarried by panel 64, and is yconnected by a shaft 86 to vernier pinions 61 -which operatively engage rack teeth 68 formed on the slide 64 as shown in Figure 3. The shaft 68 is suitably journalled in bearings 66 mounted on a fixed member 69.

The cam 46 turns in a clockwise direction as the altitude increases and is so shaped as to progressively decrease the vertical adjustmentimparted to the roller 1| per unit of rotary motion conveyed by shaft 45 as the altitude readings of the indicator increase. for example, cam 46 is positioned so that roller 1| is near its closest approach to the center of shaft 45, and upon a decrease in pressure the aneroid 65 expands upward. Since at `this point the relative movement of cam 46 necessary to lower lroller 1| to increase the force exerted by spring 41 to balance the circuit is relatively slight, the indicator recording for such a movement is However, at yrelatively high altitudes, cam 46 is correspondingly adjusted so that roller 1| is relatively widely spaced from the center of shaft 45, and considerably greater rotary movement of cam 1| is necessary to cause suiilcientl increase of the spring tension to return the slide 63 so as to balance the circuit. This increases the necessary rotary movement of the shaft 45 and causes a correspondingly greater indicator recording per unit of pressure change.

'll'his variation in the multiplication ratio of the indicator means, as the altitude increases, is necessary in order to approximate the true altitude, since, as previously explained, the vertical distance traveled for each unit of pressure fall increases with the altitude.

The operation of this portion of myinvention will now be apparent. In the normal position of the slider 63 the amount of light passing through aperture 62 and falling upon photoelectric cell 6 is equal to the amount falling upon photoelectric cell 5, and the bridge accordingly is in balance. Upon a decrease in atmospheric pressure, aneroid 65 expands, causing slider 63 to move upwardly to decrease the area of aperture |52 and thus to decrease the light falling on photoelec tric cell f5. The resistance of the cell is thereby increased, unbalancing the bridge circuit in a first direction. The unbalance potential acts through ampliner 3Q to energize winding 33 of motor di in a iirst time-phase relationship. Motor 3i operates in such a direction as to rotate cam 3G in a clockwise direction until the force exerted on aneroid t'by spring l1 has been increased by an amount equal to the decreased compressive force of the atmospheric pressure on the aneroid. lThis increase in the force of spring il .is accompanied by a compression of the aneroid, and slider 63 moves downward with it, thus increasing the area of aperture 62 until equal amounts of light fall on both photoelectric cells. The bridge circuit is vnow again balanced, and operation of moto!` 3| is accordingly interrupted. actuation of the motor has at the same time resulted in changing the 64 is adapted to be slidably ad- Thus,v at ground level reading oi member 43 altitude.

In the same fashion, upon an increase in vatmospheric pressure aneroid C contracts, causing slide to move downward to increase the area of aperture 32 and thus Ato increase the light falling on photoelectrlc cell l. v'I'he resistance of the .cell is thereby decreased, unbalancing the bridge circuit in an opposite sense to that Just described. 'The through4 a'mpliner It so that it indicates a higher to energize winding 33 o! 'motor 3| .in a second time-phase relationship unbalance potential actsl on both photoelectric cells. The bridge circuit is again in balance and operation of motor 3| is accordingly interrupted. This actuation of motor 3| has, however, resulted in change in the reading of member 43 so that it now indicates a lower altitude. I

It will be understood that the resistance of cell B is determined by the area oi aperture 62 through which light from the source 5| may pass, and that an increase of this area due to raising the upper edge by operation of knob 11 produces the same effect as an equal increase of the area due to lowering the lower edge by the operation of aneroid 85. If the barometric pressure at the reference point is greater than standard, the

static pressure at all levels also exceeds that of the standard for that level, and the aneroid is at all levels compressed beyond the amount which is proper for an accurate reading of altitude in the standard atmosphere for which the instrument is calibrated. Thisin turn means that aperture 62 is at all times o1' too great area. It will now be apparent that actuation of knob 1l to move member 64 in a downward direction has the effect of partially closing theaperture, and that by suitably calibrating the rectilinear response of aneroid 65 to changes in static pressure, the rectilinear movement of member 84 corresponding to a unit of the scale on panel 54, the graduations on indicator 43, the outline of cam 46, and the tension in spring di (the outline of aperture .62 also being considered) the indicator may be made direct reading in altitude above an arbitrary level, regardless of changes in the density of the air. The reverse of this is true if the pressure at the reerence point is below standard.

As I have previously explained in connection with Figure ll, the barometric pressure at any altitude, and therefore the pressure altitude or indication of an uncorrected altimeter, is infiuenced by the mean temperature of the air col umn between the observer and the ground. This I have illustrated in Figure 6, which shows that although the indicated altitude under high and low mean temperature conditions is the same as the actual altitude under the standard temperature conditions, the actual altitude of the observer is above the indicated altitude in the case of high mean temperatures and below it in the case of low mean temperatures.

lThus, it will be seen that under such condilo tions varying altitude indications may result at a given altitude level depending, upon the state of the air column whether contracted or expanded.

In order to obviate inaccuracies inthe indi-l cation of my instrument due to the eflect of temperature changes, Lhave provided in the form oi' my invention illustrated in Figure l (as best shown in Figure 2) a xed light control member 82 having an aperture 33 of a selected outline. A shutter 8| is pivotally attached by a suitable pivot to the xed member 32. The pivoted shutter 3| is operatively connected by a rod 58 to a free air temperature responsive device ofthe uid illled type including a bellows member 59. The lower end of member 58 has connected thereto a conduit 6G which connects remotely with a free air thermometer bulb element indicated by numeral 5 i The thermometer bulb element 6| is preferably mounted so as to contact the free atmospheric air so that the iiuid therein may respond volumetrically to the temperature of the free air atmosphere. It will thus be readily seen that as the free air temperature decreases the member 59 contracts causing the rod 58 to rotate the shutter 8| in a clockwise direction about pivot 80, so as to progressively close the slot 83 and thereby decrease the ow of light to the photo-electric cell 5.

A manually adjusable turnbuckle 92 is provided for regulating the length of rod 58. It will b realized that the iluid contained in members 59, 60, and 5| is also subject to expansion with release of pressure on bellows- 59 due to increase of altitude. The actuation of indicator 43 due to the energization of motor 3| by amplifier 39 is dependent in magnitude on the ratio between the resistance of photocells 5 and '6, that is. on the ratio of the quantitiesof light impinging upon the anodes of the two tubes. The rectilinear response of bellows 59 to changes in pressure, its response to changes in temperature, and the outu line of aperture 83 are so selected that if the length of rod 58 is adjusted by means of turnn buckle S2 to bring shutter 8| flush with the bottom of aperture 83 when the instrument is under standard conditions at the reference point, the relative occlusion of apertures 83 and 62 with rise of the craft through a standard atmosphere causes uniform increment in the indications of indicator 43. For a rise of the instrument through an atmosphere having a temperature gradient other than standard the relative occlusion of aperture by shutter Si is deter mined as before by the joint effect of temperature and pressure changes on taking place a different rate so theta factor proportional to the departure of the temperature gradient through which thecrait is rising from the standn ard gradient is introduced into the signal elect ing actuation of indicator It will further be noticed that this factor occurs independent of and may occur simultaneously with change in the factor iniiuenced by variation in pressure, controlled by member G3 and aperture di?.

The purpose of turnbuclrle t?? is to bring; the upper surface of shutter di flush with the lower portion of aperture d3 in the beginning oi the rise of the craft, so that there will be no period during which the shutter is moving with respect to member 82 and yet not'varylng the occlusion of aperture 83. Not only the temperature gradient through which a craft must rise from a given reference level, but the temperature at the starting level may vary from the standard. The

mechanism I have just described has been provided to permit correction of the instrument for.

variations in gradient, but I also provide means for correcting the indications of the instrument for variations in the temperature at the reference level. For this purpose there is provided a slide 88 having mounted thereon rack teeth 8'! operably engaged by a pair of pinions 85. The pinions 85 are' connected to a shaft 84 journalled in bearings 44 mounted on the member 82, the shaft 84 projecting through a suitable control panel 54. Mounted at the free end of the shaft 84 is an adiustmentknob 48 having provided a pointer 42 for cooperation with suitable scale markings on the control panel 54. Thus, by adjusting the knob 48 .the slide 88 may be raised or traction 'of the gas mass or free atmospheric air column resulting from changes in the temperalowered with respect to the slot 83 and-the i'low of light to the cell Sthrough the slot 83 thereby adjusted.

It will be understood that the resistance of cell 5 is determined by the area of aperture 83 through which light from source 5| may pass, and that'an increase of this area due to raising the-upper edge by operation of knob 48 produces the same effect as an equal increase of the area due to lowering the lower edge by operation of Y bellows 58. 1f the temperature at the reference pointis above the standard, bellows 59 is expanded and shutter 8|, if adjusted to the lower edge of aperture 83 under standard conditions,

has now taken a position below the edge of aper ture 8| in an effort to still further increase the' area for light flow.

Movement of shutter 8| below the point at which lit coincides with the bottom ofl aperture 83 introduces error into the device, since it makes no actual change in the area through which light may pass to impinge on cell 5. It is necessary that the shutter have that position of coincidence with respect to aperture 83 at the beginning of a flight. Therefore turnbuckle 92 is adjusted to set shutter 8l in coincidence with the bottom of the slot, and the area of aperture 83 is increased by actuation of knob 48 to provide the necessary ratio of light impinging on cell 5V and 6 respectively. This light ratio acts through the bridge and amplifier toenergize motor 3| in such `fashion as to move indicator 43 to a position consistture thereof above or below an assumed standard value.

As is well known in the art and graphically shown in Figure 11,l the vertical distance traveled for each unit of barometric pressure fall increases with the altitude. Therefore, in order to secure a scale evenly divided in altitude units the ratio of the movement of the indicator means 43Ito the deflection` of the pressure responsive element 85 must vary continuously with the altitude. For this purpose'the cam 46 is so shaped that the movement of the cam it required for unitary increase in the tension in spring 41 increases as the indicated altitude increases so that the ratio between unit increase in altitude indications and unit of atmospheric pressure fall is properly increased with the altitude.

This is specifically accomplished by constructing the cam 46 so that the pitch of the actuating cam surface decreases in proper proportion for the assumed standard of operation as the cam is turned in response to a decrease in atmospheric pressure or rise in altitude. Thus as the altitude increases the rotary movement of the cam 48 necessary to return the aneroid to the 'starting or zero position is increased per unit of pressure limits the passage of -`light to photoelectric cell ent with the departure of temperature from the standard, and this corrects the instrument for temperature variations at the reference point.

It will be appreciated that an increase in the illlnnination falling on photoelectric cell 5 (which lowers its resistance) is eiectively the same in the bridge circuit asa decrease in the illumination of photoelectric cell 6 (which raises its resistance) and that the opposite is also true. r

VAs the temperature decreases the shutter 8| pivots upward so as to progressively close a portion of the slot 83 thereby decreasing the light supplied to the cell 5. If the decrease in temperature is at a rate greater than that in the standard atmosphere, this decrease in light is of such magnitude as to unbalance the bridge in the same sense as motion of member 03 .downward: that is, motor 3i is actuated to decrease the reading of the indicator and to reduce the force of 'Spring 4l on aneroid 65. A decrease in temperature thus hasv the same effect onl the bridge circuit and the motor as an increase in pressure due to a decrease in altitude.' 4An increase in temperature has the opposite eiect.

The 'introduction of these factorsinto the read- 6 that the bridge circuit 35 is balanced.

Figure 11 shows that the barometric pressure at an altitude of 25,000 feet in a standard atmosphere is 278 millimeters of mercury: in the atmosphere of lower mean .temperature the pressure is 290 millimeters, while in that of higher mean temperature the pressure is 255 millimeters.

An uncorrected altimeter is calibrated in terms of the standard atmosphere, and therefore reads 24,250 feet in the colder atmosphere and 27,500 feet in the warmer atmosphere, although the plane is actually at an altitude of 25,000 feet in each case. In order to bring about a correct reading of my altimeter in the warm atmosphere,

.my spring -46 must increase its pressure upon aneroid 85 by an amount equal to the pressure differential between 27,500 feet and25,000 feet or, 23 millimeters of mercury. The aneroid is actually working in the 27,500 foot range while the indicator and therefore cam i6 is Working in the 25,000 foot range. Similarly, in order to bring about a correct reading of my altimeter in the colder atmosphere, my spring e5 must decrease its pressure upon aneroid by an amount equal to the pressure dierential between 25,000

feet and 24,250 feet: that is, by l2 millimeters of V mercury.

cury. In a colder atmosphere it `is 278 milings of the instrument corrects the instrument Y for errors introduced by the expansion 0l' 09nlimeters, and in a Warmer atmosphere it is 242 millimeters. The change in pressure in thestandardatmosphere is 10 millimeters. In a colder atmosphere the change is 12 millimeters while in the warmer atmosphere the change is 13 Vmillimeters. The indicator of my compensated altitude must'ndiove through a distance proporas previously explained, as the temment of the shutter 98. y provided on the knob 55 for cooperation with a less of the atmosphere. My temperature compensator, therefore, alters the operative standard l of the altimeter so that the same change in indicated altitude takes place whether the change in barometric pressure be millimeters in an atmosphere of standard temperature, 12 millimeters in a warmer atmosphere, or 13' millimeters in a colder atmosphere.

In certain simplified applications it may not be desired to have the device include both responsive and manually operable members. In such an application a modified form of means for controlling the light supplied to photo-electric cell B may be provided such as shown in Figure 4, wherein-a fixed light control member 95 is provided having formed therein an aperture 96. Ailixed to a. shaft 91 pivotally connected to the member 95 is a shutter S8. Mounted on the shaft 91 is also a gear 99 intermeshing with a second gear 14 aiiixed to one end of a control shaft 15 which is journaled in a, bearing arm 49 projecting from the member 95: said shaft 15 passes through a suitable aperture formed in the control panel 54. A control knob 55 is mounted on the shaft 15 for addust- A pointer 56 may be suitable indicator scale as shown in Figure 5. The member 95 and control shutter arrangement may be readily substituted for the temperature responsive control shutter for controlling the illumination passing from light 5I to the photoelectric cell 5 the amount of light passing through the aperture 96 being controlled by the position of the shutter 98.

An altimeter of the type embodying manually adjustable control means such as shown in Figure 5 is readily adapted for use Where accurate correction factors are available.

Thus, for example, if a ground operator in a commercial air line organization nds that in order to correct the ground altimeter so as to indicate the actual ground altitude at the ground level it is necessary to adjust the position of knob 11, such information can be conveyed by radio to the pilot of the plane. Similar information regarding the temperature at the ground station may be given to the pilot in the same way. The information supplied to the pilot enables him to operate knobs 418 and 'El so that suitable correction for change in the atmsopheric condition at the bottom of the column of air may be made. The altimeter then operates at an assumed standard based upon actual ground atmospheric conditions rather than upon an assumed standard based upon an assumed ground atmospheric condition. lThus, at the altitude to be measured margin of error isrelatively slight since the indicated altitude is in error at Ythe point of measurement only insofar as the atmospheric conditions at such. point varies from the known ground atmospheric conditions rather than from an assumed ground atmospheric condition. Further, if desired, the reading of the altimeter may be corrected for temperature conditions existing at the upper level or point of flight or for the difference between the ground temperature and the temperature at the flight level.

ln a further form of my invention illustrated by Figure 7 there is provided a variable reluctance or impedance bridge circuit indicated generally by the numeral itil and including variable resistances Hi and lili having a common terminal I3l and impedance coils lit and ttt havtemperature responsive.

ing a common terminal |30. 'I'he resistance III is connected'to the impedance coil II3 by a conductor |I8 While the resistance II2. is connected to the impedance coil |I4 by a conductor H9.

A pair of terminals |32 and |33-v are located on conductors H8. and IIS, and constitute the input terminals of the bridge circuit.A The bridge is energized from the secondary winding |22 of a :transformer |23, to which connection is made by conductors and I 2|'.

Current is provided in secondary winding |22' by transformer action from primary winding |24, which is supplied with alternating current from any suitable. source by conductors Iiand I 26.

Terminals and l3-I comprise the output terminals yof Abridge circuit H0, and are connected as by conductors I 21 and |28 with input terminals |36 and |35 of an amplifier I25lhaving output terminals |36 and |31.

It will be readily seen that the coils II3 and H4 form two adjacent arms of the variable im.- pedance bridge circuit I|0. The variable resistance windings l I| and I i2 form the opposite arms of the bridge circuit I I0. v

Coacting with the coils II3 and II4 is a movable core II5. The core I l5 of magnetic material is operably connected by a rod IIS to an atmospheric pressure responsive member or aneroid II1.

The bridge circuit l I0 is so arranged that normally with the core I I5 positioned substantially equally within the two coils I I3 and I I0 the vbridge circuit IIO is balanced and no potential appears across output terminals I 30 and I3I.

However, upon a decrease in atmospheric pressure permitting the expansion of the aneroid II1, the core IIS moves upward. The core II5 then approaches further toward or extends further into the coil I i3 and is correspondingly removed further from or project less into the other coil I I4. The magnetic flux threading the coil I I3 therefore increases, increasing the impedance of the coil H3, and the flux through the coil llt decreases, decreasing the impedance 0i the coil IM, thus unbalancing the bridge circuit in such fashion that a potential diierence appears between terminals IM and having the same creasing so as to cause the contraction of the aneroid lil and the downward movement of the core liti past the balancing point of the bridge circuit itil, the impedance of the coil it? is decreased and the impedance of the coil inn creased unbalancing the bridge circuit in such fashion that a potential difference appears betweenterminais and iSE having the saine polarity and til efphase relation as the potential difference between terminals it@ and ritt, and also of a magnitude proportional to the unbalance of the bridge. This potential difference is then reddirectly to terminals i3@ and E35 of amplifier t2@ Since the instantaneous potential diierence between conductor ital and terminal is out of phase with that between conductor H9 and terminal ilii, it Will be seen that the potential difference impressed across terminals it and S35 of ampliier l2@ when the impedance of coil tit is increased is that impressed on the amplifier input when the impedance of coil ||4 is increased. It should also be pointed outthat anincrease in the impedance of c'oil'- |'f'l'3" iectively the same-as a decrease in tneimpedri recu m. i be any suitable type of 'Inefajrnpriner |529 may' electronic 7 aiplif-ier well? 'known in the yart such as shown or example in the previouslyn'oted PatentrNc:51586233; granted May 25.21926, to

Anschutz-Kaeinpfe, or may be preferably an electronic amplifier ofthe `type shown and 'described ofthe cam |58.

in the; aforenoted co-pending application 'of Alherring-fuma, serial No. 437,561, dated Aprile;

Winding |41 is connected bythe conductors |43 and |44 to thevoutput terminals |36 and |37 of the amplifier |29,'j.

A transformer |4|'is provided for continuously energizing the motor winding |4|i,saidV transformer having a primary winding |40 and a secondary winding |42. The primary winding |40 is connected 'through conductors |38 and .|39 to conductors and |26 which are in turn connected to a suitable source of alternating current.

Current is supplied to the secondary'winding |42 through transformer action from the primary winding |40. Motor winding |46 is continuously energized from the secondary winding |42 of the transformer |4|. A condenser |48 is connected in series between the secondary winding |42 and the motor winding |46 and causes a p hase displacement of substantially 90 between the current owing inthe winding |45 and the terminal voltage of the secondary winding |42. Y

As described in detail in the co-pending Up. ton application previously. referred to, the amplifler 29 is of a type which produces in the ybranch of the output circuit including conducshaft |50, train of gears |5| and shaft |56, to turna cam |58 so as toV adjust the tension of a loading spring |59 for a purpose which will hereinafter be explained.` The `cam |58 cooperates with a roller member 4| 60mounted on the loading 4spring |59 for th adjustment of the tension of the spring |59. The loading spring |59 is plvoted at one end cna xed knife edge |62 and engages'v at the oppositefjend theaneroid exerting a force 'thereon vd`ef ae'r i ent upon the adjustment nIt] will be seen that upon a decrease in pressure; aston a'rise Ain altitude, the aneroid ||1 expands forcingthe rod H6 upward against' the tension of the spring |59. Moreover, the upward movement of the rod ||5 likewise moves the core H5 upwardl which as previously explained increases the-impedance of the coil ||3 and decreases the impedance of the coil ||4 and thereby cause an unbalancing of the variable reluctance bridge circuit ||0. The unbalanced potential is amplified by the Vamplifier |29 in a manner previously described and the thus amplied electromotive force drives the motor |45 so as to rotate the shaft |50 in a direction to-cause the actuation of the indicator means |52 in a manner well' known in the art so as to indicate an increase in altitude. Moreover, the motor also, upon such unbalancing of the bridge circuit ||0, turns the shaftA 56 through the shaft |50 and intermeshing gear means I5I to turn the cam |58 in a clockwise direction so as to cause downward movement ofthe roller |60 thereby increasing the tension exerted by the spring |59 on the aneroid ||1. This in turn causes the contraction of the aneroid l1 under the atmospheric pressure and the additional tension of the loading spring |59 to cause A downward movement of the rod I6 until the core tors |43 and |44 and motor winding |41 an alternating current of the same phase and frequency as the potentiall applied to input terminals |34 and |35. Since transformer primary windings |24 and vare both connected to the same alternating current supply lines |25 and |26, it will be understood that the potential appearing at the terminals of the secondary windings |22 and |42 'are of the same phase.

The current liowing through winding |46 is, howeveryshifted substantially 90 electrical degrees with respect to this potential by the condenser |48, as previously explained. The Winding |41, on the other hand,

vis supplied with current either in phase or 180 out of phase with the :terminal potential of windings |22 and |42 depending-upon the direction of unbalance of the bridge circuit it.

In accordance ,with well known characteristics of a split phase motor, when the windings |48 and |41 are supplied with currents 90 out of phase, the motor is caused to rotate in a predetermined direction. The operation of a split phase .motor of the type described herein is described in the co-pending Uptonapplication previously referred to.

The motor |45 is adapted to drive a shaft |53 in a direction depending upon the unbalancing of the bridge circuit i0 and the resultant current supplied from the amplier to the motor |45. The shaft |is, in turn, adapted by suitable means such as the gear train |5| to drive an indicator means |52, which is of a suitable type well known in the art. Moreover, the motor |45 is further adapted by suitable means such as the ||5 is once' again positioned in such a manner as to balance the bridge circuit ||0, whereupon the motor |45 ceases actuating caml |58 andthe indicator means |52.

Upon an increase in atmospheric pressure, as

in the case of a decrease in altitude, the aneroid ||'|l contractsv causing a downward movement of the rod IB and causing the core I I5 to increase the impedance of coil |4 and decrease the impeda'nce of coil I3. The amplified unbalanced potential of the circuit ||0 now `causes rotation of the motor |45 in a direction opposite to that in the preceding paragraph, moves the indicator means to indicate the decrease in altitude and rotates the cam |58 ina counter-clockwise direction so as to decrease the tension of the loading lspring |59. The decreasing tension of the spring |59 permits expansion of the aneroid ||1 and upward movement of the core ||5 to a position bal- A ancing the bridge circuit ||0, whereupon theunbalanced potential ceases and the flow of amplified current to the motor |45 terrr'iinates. The

movement of the motor -|45 thus stops until the bridge circuit H0 is once again unbalanced.

Cam |58 turns in a clockwise direction as the indicated altitude increases, and the function of this cam is the same as that of cam 46 clearly set forth previously.

In order to more accurately adjust the altimeter lby taking into account the barometric pressure at the ground and for variations from the standard mean temperature at the upper level, I have provided the adjustable resistances ,i ii and H2 which under assumed standard starting or ground-level conditions are of equal resistance.

y The variable resistance is adapted to be operated by a-knob |0|, as shown in Figure 8, so

@that upon a decrease in barometric pressure vbelow the assumed standard for the ground or starting level, the resistance I may be increased in accordance with a predetermined adjustment scale and thereby cause the unbalancing of the bridge circuit IIO, and an unbalance potential therefore appears across terminals |34 and |35. This unbalance potential ampliiied, as previously explained, then causes the motor |45 to rotate in a manner previously described so as to decrease the altitude readings and to turn the cam |58 in a counter-clockwise direction so as to decrease the Aratio between change in indicator readings andpressure change, and to decrease the force exerted by the loading spring |59 on the aneroid I 1. The decrease in tension of spring |59 in turn causes the core ||5 to rise until the impedance of coil II3 is increased and the impedance of coil 4 decreased to a point whereupon `the bridge circuit is once again balanced. Thus, correction is affected for altitude readings twhich if uncorrected would under an assumed indicator standard read too high duev to a decrease in the pressure of the air column at the ground or starting level of the measurement.

Further, upon an increase in the ground barometric pressure causing the altimeter at the origlnal standard of operation to indicate an alti- Y tude below tthe true altitude, the resistance III may be decreased by adjustment knob I0| in accordance with a predetermined adjustment scale as shown in Figure 8, causing the unbalancing of the previously balanced bridge circuit I I0 so that the current due to the unbalance potential flows through the resistance III and coil ||4 causing in turn the motor |45 to rotate so as to turn the indicator |52 to increase the indicated altitude. The motor |45 simultaneously turns the cam |58 in a clockwise direction so as to increase the ratio between pressure change and change in indicator readings, and to increase the tension in spring |59 whereupon the atmospheric pressure acting upon the aneroid II'I together with the increased compressive force of spring |59 tends to force the core ||5 downward until the impedance of coil ||4 is increased and the impedance of coil I3 decreased to a ,point whereupon the bridge circuit |I0 is once again balanced. Thus the standard of operation of the altimeter may beadjusted for the actual pressure conditions existing at the ground or starting level of the measurement.

It will be obvious that the effect of varying the resistance `of the resistor II| in this modification of the device is the same as the effect of varying the setting of knob 'I1 in my first embodiment. In a similar fashion, varying the resistance of resistor I|2 in the present modification has the same unbalancing effect on bridge circuit IIO as varying the settingof knob 48 or knob 55 in my rst embodiment.- A decrease in the resistance of the resistor ||2 unbalances bridge circuit ||0 in the same direction as an increase of the impedance of winding |I4,v energizing the motor to operate the indicator'and rebalance the bridge by means of core I |5. A knob |02 is provided for adjusting the resistance of resistor II'2. and the knob may have a pointer for cooperation with graduations suitably calibrated in temperature units to give correlated interrelation between actuation` of knob |02 and the remainder of the device, these gradnations being carried by a suitable control panel; It will thus be seen that in the form of my invention illustrated in Figure 7, I have provided 18 separate manually operable electricalv control means for adjusting the altimeter for ground atmospheric pressure and for changes in the temperature of the free atmospheric air whereby the return or zero point of the aneroid may be adjusted and the multiplication ratio of the indicator means varied. Thus, the standard of operation of the altimeter may be adjusted so as to operate accurately witha minimum of error under conditions other than those `ol the standard to which the instrument was calibrated. In the form of my invention disclosed in Figure 7, if the temperature gradient of the air column approximates that of the assumed standard. and the above described'adiustment is made for ground temperature deviation from the standard, the cam arrangement I 58 and spring |59 are so constructed as to increase the multiplication ratio of Y the indicator means as altitude increases. The instrument thereby properly reilects an increase in vertical distance traveled per unit of pressure as the altitude increases.

However, vunless further compensated, such a1- titude readings may depart from the true altitude insofar as the temperature gradient or mean temperature of the actual air column diiers from that of the assumed standard atmosphere. The extent of such error as shown in Figure 11 is substantially negligible at the relatively lower altitudes but the error increases with the altitude and represents a substantial error particularly at the greater heights.

In order therefore, to compensate for such errors, I have further provided in the form oi.' my invention illustrated in Figure 9 automat-ic temperature correcting means. vided an electric thermometer or resistor |80 of a metal lsuch as nickel having a positive temperature coeflicient of resistance. The resistor |80 is mounted so as to be directly affected by the temperature of the free atmospheric air.

'Thus, upon the temperature of the free atmospheric air decreasing the resistance of the member will be reduced, and similarly upon an increase in the temperature of the free atmospheric air the resistance of the member |80 will be increased. A micro or milli voltmeter IBI is shunted across the resistance |80 and is directly aiected by the variations in the resistance thereof caused by the rise or fall of the temperature of the free atmospheric air. The meter |8| has a scale graduated so as to indicate degrees of temperature oi the free atmospheric air acting upon the resistor 80. A switch |03 is provided for connecting the meter IBI in and out of the circuit.

Resistor |80 is connected in series with a variable resistance |83 which may be formed of a material such as manganin the electrical resistance of which is relatively unaffected by temperature change. Resistors |80 and |83 form one arm of a bridge circuit |82 which is otherwise essentially'the same as the bridge circuit ||0 previously shown in Figure 7 wherein like numerals indicate similar parts. The resistance of the variable resistance |83 is so arranged that at the ground or starting level the same may be manually varied by adjusting the knob |05, Figure 10. The knob |05 has a pointer |08 cooperating with suitable scale markings whereby the resistance |83 may be increased proportionately as the temperature of the atmospheric air at the starting level decreases below the assumed standard causing in response thereto the re Thus, I have prov creases.

temperature of the free atmospheric air de- Thus, the resistance |83 compensates for `the other variable resistance |88 at the ground level. Similarly theresistance |83 may .be manually adjusted so as to decrease propor- 20 the temperature at the lower and upper level of measurement.

Although several embodiments of the ment of the parts without departing from the spirit and scope of the invention as the same sistance |l| in the manner previously described -with reference to the form of Figure 7. However, durlng flight the resistance |83 may be manually adjusted by the knob |05 in accordacce with suitable scale markings such as shown in Figure 10 for actual atmospheric temperature at the ground level, which information may be conveyed to the operator by radio or other suitable means.

However, upon the altimeter being raised by an airplane `or other suitable means through the air column, the resistance |83 after adjustment to -ground temperature conditions remains constant as it is not subject to variation in the temperature of the atmospheric free air, while the resistance of the thermometer resistor I8@ varies with the temperature of the free atmospheric air, thereby varyingthe resistanceof the arm of the bridge circuit |83 including the resistances |83 and |80. However, since the resistance |83 remains constant after adjustment to ground atmospheric temperature and the resistor l|8il changes in accordance with the temperature oi the atmospheric free air, it will be readily seen that the eiect of such arrangement is to continuously adjust the operative standard of the altimeter while passing through the column of air for an average temperature between that existing at the lower end of the column of.' air and at the point of measurement. It will be apparent that the function performed by resistor |80 in the present modification of my invention is strictly analogous so that performed by member 8| and so forth in the modification disclosed by Figure 1, and that members 8| and 86 of Figure 1 together are the equivalent of members |85 and |83 in the present modication.

In summary, it Will be seen that I have provided novel compensating means for an altimeter whereby the-calibration of the instrument may be adjusted at any time to accord with the atmosphere through which the craft is iiying. I'he device operates Without adjustment in a Vstandard atmosphere, and is adjustable, in part automatically and in part manually, `to accord perature distribution in the column of airv through which the apparatus passes based 119011 Awill now be understood bythose skilled in the art. For a definition of the limits of the invention, reference will be had primarily to the ap-v pended claims. v`

I claim as my invention: 1. An altimeter comprising, in "combination,

anatmospheric pressure responsive means, an altitude indicatingV means, adjustable trans-- mission means for driving the 'indicating means in accordance with the response of the atmospheric pressure responsive means, and means for adjusting the transmission means whereby the rate of change in altitude indications for unit of atmospheric pressure change maybe varied, said adjusting means including a first and second 'manually operable means to regulate Said operative standard in accordance with first and second atmospheric conditions existing at the vstartv ing level of measurement and third means to regulate vsaid operative standard in accordance with the variation, from an assumed standard, of an atmospheric condition existing at the level oi measurement.

` 2. An altimeter comprising, in combination, an atmospheric pressure responsive means, an altitude indicating means, adjustable transmission means for driving the indicating means in accordance with the response of the atmospheric pressure responsive means, and means for adjusting the transmission'means whereby the rate of change in altitude indications for unit of atmospheric pressure change may be varied, said adjusting means including a manually operable -means to regulate said operative standard inaccordance with atmospheric pressure conditions existing at a starting level-of measurement and further means to regulate said operative standard in accordance with temperatures of the free atmospheric air existing at the starting level and at the level of measurement.

3. An altimeter comprising, in combination, an atmospheric pressure responsive means, an altivtude indicating means, transmission means for driving the indicating means, adjustable in' accordance withthe response of the atmospheric pressure responsive means, and means for adjusting the transmission means whereby the rate or change in -altitude indications for unit of atmospheric pressure change may be varied, said adjustingmeans including first manually operable means to regulate said operative standard in accordance with atmospheric pressure conditions existing at a starting level of measurement, second manually adjustable means to regulate said operative standard in accordance with the temperature of the free atmospheric air existing at the starting level of the altitude measurement, and third means for iniluencing said operative standard in accordance with the temperature ,of the free atmospheric air existing at the level-of the altitude measurement, said second and third means arranged in such a manner as to regulate said operative standard in accordance with the arithmetic mean of the temperatures oi the free atmospheric air at the starting level and at the level of the altitude measurement.

inven-A,l

`tion have been illustrated and described injde-f.v tail, it is tobe expressly understood that the Ain-y vention is not limited thereto, and that various1 changes may be made in the design and arrange- 2l y4. An altimeter comprising in combination an atmospheric. pressure responsive means movable from a lrst position to a second position, a counter biasing means acting upon the atmospheric eratively connecting said responsive means with said resetting mechanism in controlling relation thereto, said last named means including first and second manually operable means for actuating the resetting mechanism so as to vary the said return position of the measuring member, iirst visual indicating means whereby the position of said first manually operable means may be correlated With specific values of atmospheric pressure, and second visual indicating means whereby the position of said second manually operable means may be correlated with specific values of atmospheric temperature.

5. An altitude measuring device comprising in combination, a plurality of electrical resistors forming an electrical bridge circuit, two of said resistors being adjustable for atmospheric temperature and atmospheric pressure conditions at the starting level of the altitude measurement, another of said resistors responsive to atmospheric temperature and having electrical characteristics variableby changes in atmospheric temp'erature, a slidably mounted magnetic core surrounded by inductors in opposed legs oi said bridge circuit, an atmospheric pressure responsive member operatively `.connected to said core whereby upon movement of said atmospheric pressure responsive member movement is imparted to said core in such a manner as to vary the impedance in said inductors in the opposed legs of said bridge circuit, a spring means biasing the pressure responsive member, adjustment means for varying the tension of the said spring means, motor means energized upon the unbalancing of said bridge circuit for adjusting the said adjustment means so as to vary the tension of the spring means so as to cause thereby -the movement of the said member adjusting the core for rebalancing the bridge circuit, andan altitude indicating device actuated by the said motor means in response to the movement thereof,

whereby the altitude indicating device is positioned in response to the atmospheric pressure existing at the point of measurement and the position of said indicating device corrected for the temperature to which said atmospheric temperature responsive resistor is subjected at the time of measurement and for atmospheric pressure and temperature conditionsexisting at the starting level ofsaid measurement.

6. A pressure responsive instrument, comprising, in combination, a balanced bridge circuit, a pair of adjustable members for unbalancing said circuit, a motor means energized upon the'unbalancing of said circuit, an atmospheric-pressure condition responsive means operatively connected to one of said members and a ltemperature condition responsive means operatively connected .to the other of said members, both said condition responsive means jointly controlling Ithe unbalancing of said circuit, indicator means actuated by said motor means upon the unbalancing of said circuit, and biasing means actuated by said 'motor means for moving at least one of said members so as to rebalance said circuit.

7. An instrument comprising, in combination,

a bridge circuit, a plurality of adjustable memybers for controlling the electrical balance o! said circuit, a first condition responsive means and a secondcondition responsive means, operating the said adjustable members in such a manner as to unbalance the circuit, indicator means, means actuating said indicator means upon the unbalancing of the circuit, biasing means effective upon actuation to alter the response of one of said condition responsive means, and means actuatingsaid biasing means, upon unbalancing of Ithe circuit, in such fashion as to alter the responseof the last named condition responsive means to rebalance the circuit.

8.' An instrument comprising in combination a measuring member movable from a iirst position under influence oi the forces to be measured, a biasing means acting upon the measuring member, a sourceof light, a plurality of photo-electric cells arranged yto be equivalently illuminated from said source, whereby to induce equivalent electrical responses from said cells, control means actuated upon movement of said measuring member for varying the ratio of light supplied to said cells in such a manner as to differentially affect said responses of said cells, mo- .tor means controlling the actuation of said biasing means, and means causing energization of said motor means in accordance with differential responses of said cells, whereby to actuate said biasing means in such a. manner as to cause the return of -the measuring member to the iirst position. f

9. An instrument comprising in combination a measuring member movable from a rst position under influence of the forces to be measured, a biasing means acting Jupon the measuring member, a source of light, a plurality of photo-electric cells arranged to be equivalently illuminated from said source, whereby yto induce equivalent electrical responses from said cells, irst control means actuated upon movement of said measuring member for varying the ratio of light supplied .to said cells in such a manner as to differentially aiect said responses of said cells, motor means controlling the actuation of said biasing means, means causing energization of said motor means in accordance with differential responses of said cells, whereby .to actuate said biasing means in such a manner as to cause the return of the measuring member .to the iirst position and second control means for varying the ratio of light supplied to said cells so as to cause said motor means to change 4the return position of the measuring member.

l0. An altimeter comprising in combination, a normally electrically balanced bridge circuit, a single source of light, a iirst photo-electric cell, asecond photo-electric cell, said photo-electric cells forming adjacent counter balancing arms of said bridge circuit, said cells of the electron emissive type arranged to receive iight from said source, a ilrst apertured control slide for regulating the intensity of the light supplied to the rst cell from said source. a second apertured control slide for regulating the intensity of the light supplied to the second cell from said source, said control slides thereby regulating the electron emissions from said cells and thus the balance of said bridge circuit, an aneroid for movably actuating said ilrst slide upon a change in atmospheric pressure, a spring exerting a tension upon said aneroid, a mechanism controlling the tension of said spring, an altitude indicating device, a free air atmospheric temperature responsive theri 23 mometer bulb of the distant type for movably actuating the second slide upon a change in temperature, and a reversible electronic drive motor controlled by the unbalanced potential oi said bridge circuit and mechanically connected to said tension control mechanism and said indicating device, and upon movement of either of said slides causing the unbalancing of said bridge circuit energizationof said motor is effected in such a manner as to adjust said indicating device and actuate said tension control mechanism so as to vary the tension of said spring causing the movement oi' said aneroid and movement of the rst sllde'so as to balance the said bridge circuit, whereby the altitude indicating device is positioned in response to the atmospheric pressure existing at the point of measurement and the positionl of said indicating device corrected for the temperature to which said temperature responsive device is subjected at the timeof measurement.

l1. An instrument comprising in combination: an atmospheric pressure responsive member movable from a rst position under the influence of the forces to be measured; a counter-biasing means acting upon said member; a normally balanced electrical bridge circuit; means associating said member with said bridge circuit, whereby movement of said member from said rst position bridge circuit for actuation upon unbalance ofv said bridge circuit to vary the force exerted bysaid counter-biasing means for returning said member to said first positionindicator means movably positioned by said resetting mechanism;

and manually operable means for unbalancing Y I said bridge circuit to bring about actuation of said resetting mechanism so as to vary the re- .turn position of said member andthereby adjust the position of sai-d indicator means.

12. In an altimeter the combination with a resilient diaphragm movable from a rst position in response to a change in atmospheric pressure acting thereon, 'of atension spring acting upon said diaphragm, a source of light, a plurality of photoelectric cells arranged to be equivalently illuminated from said source whereby to induce equivalent electrical responses from said cells, a iirst .control device actuated upon movement of said diaphragm forvarying the intensity of the light supplied to said cells in such a manneras to diiferentially affect the electrical responses of said cells, `a motor controlling the force exerted by said tension spring, and means causing energization of said motor means in accordance with differential responses of said cells, whereby to actuate said spring to cause the return of the diaphragm to the iirst position, a second control device for varying the intensity of the light supplied to said cells so as to di`erentially affectthe elec-" trical responses from said cells in such a manner as to cause said motor to change said rst position of the diaphragm, and a free air atmospheric temperature responsive device for actuating the means, first control means actuated upon movement of said measuring member for varying the ratio of light supplied to said cells in such a manner as to differentially aiect said responses of said cells, motor means controlling the actuation of said biasing means, and means causing energize.-

tion of said motor means in accordance with dif-V ferential responses of said cells, whereby to actuate said biasing means in such a manner as to cause the return of the measuring member to the first position, said motor means adjusting said in.. dicator means during such return movement of said measuring member, and second control means for varying the ratio of light supplied to said cells in such a manner as to cause said motor means' to change said return positionfof the measuring member and cause said motor to change the adjustment of said indicator means.

14. In an altimeter the combination with a resilient diaphragm movable from a i'lrst position in response to'a change in atmospheric pressure act- -a manner as to differentially ailect the electrical responses of said cells,- a motor controlling the force exerted by said tension spring, and means causing energization of said motor means in ac cordance with differential responses of said cells, whereby to actuate said spring to cause the return of the diaphragm to the rst position, and said motor adjusting said indicating-device during such return movement of said diaphragm, a second control device for varying the intensity of the light supplied to said cells so as to diiierentially aiect the electrical Vresponses from said cells in such a manner as to canse said motor to change said rst position of the diaphragm and cause said motor to change the adjustment of said indicating device, and a free air atmospheric temperature responsive device for actuating the second control device.

l5. An altimeter comprising .in combination: movable means "-whose position is related to atmospheric pressure; movable means whose position is related to altitude indications on a .xed

scale; adjustable' means operatively connecting said first named movable means and said second named movable means, whereby movement of said rst named lmovable means may cause movement of said second named movable means, said adjustable means comprising means for varying the ratio between movements of said connecting means and of said second named movable means,

said ratio varying means comprising rst manually operable control manseffective to alter the rate of change of said altitude indications with changes in atmospheric pressure, and a second manually operable controlmeans eective to alter the rate of change of said altitude indications with changes in atmospheric temperature; iirst calibrated means whereby actuation of said rst control means may be interpreted in terms of deviation of actual atmospheric pressure at a reference level from a standardyand second calibrated means whereby actuation of said second control means may be interpreted in terms of the difference between the temperature ambient to said rst named movable means andthe temperature at said reference level.

i PAUL F. SHIVERS. 

